In oil field cementing operations a mixture of water, cement and particular additives are pumped down through the steel casing into the well bore and back up through the annulus around the casing to its desired position. Such action is known as primary cementing. The principal purposes of the primary cementing of oil and gas wells are to restrict fluid movement between formations and to bond and support the casing. To be useful as an oil field cement, a cementing composition must be designed to allow adequate placement time and have setting properties designed to resist gas leakage, loss of strength, and corrosive environmental elements. As the bottom hole circulating temperature (BHCT) of a well increases, slurry retardation and viscosity become more critical due to their effect on pumpability and compressive strength.
The present invention relates to cementing compositions and methods of cementing oil and gas wells and the like having a BHCT greater than about 200.degree. F. using such compositions. More particularily, the present invention concerns incorporation of particular terpolymers and water soluble borates in hydraulic cement for the purpose of retarding the set time of such cement during cementing operations. The terpolymers of the present invention are comprised of about 10 to about 65 mole percent of a strong acid monomer selected from the group consisting of sodium vinylsulfonate, 2-acrylamido, 2-methyl propane sulfonic acid and vinyl benzene sulfonic acid; about 10 to about 60 mole percent of a weak acid monomer selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid and vinyl acetic acid; and about 10 to about 60 mole percent of a nonionic monomer selected from the group consisting of acrylamide, N-vinylpyrrolidone and N,N dimethylacrylamide.
Certain polymer compositions have long been recognized by those skilled in the art of cementing wells in the petroleum industry as cementing additives useful in retarding the set time of cement and water. Such compositions are commonly referred to as "cement retarders." For example, lignosulfonic acid salts such as calcium lignosulfonate, are commonly used as cement retarders. However, such retarders are effective in BHCT temperature ranges up to only about 200.degree. F. Additionally, many cementing compositions contain large amounts of salt. Salt causes cement to expand and also acts as a dispersant. Salt cement slurries help prevent sluffing when used with respect to shale and other salt-containing formations, as well as helping to prevent bridging and lost circulation. However, such salts tend to wash out the lignosulfonate thereby negating its set retardation characteristics.
When either salt cement slurries or fresh water cement slurries are prepared for cementing subterranean wellbores, it is often desirable to include various additives to tailor the cement slurry to specific well requirements. Such additives include accelerators, retarders, dispersants and fluid loss additives among others. Some of these additives have an effect on the properties of other additives and such effects must be taken into account when preparing the overall slurry.
To improve the performance of the lignosulfonic acid salts at higher temperatures, organic acids, such as gluconic acid, have been added. Further, as disclosed in U.S. Pat. No. 3,748,159, pentaboric acid salt has been combined with a lignosulfonic acid salt to produce a cement retarder for high temperature application. However, use of a lignosulfonic acid salt-organic acid-pentaboric acid salt containing composition may only be effectively used up to a temperature of about 450.degree. F. As such temperature is approached, the amount of retarder needed to retard the cement for a sufficiently long period of time to allow placement is of such a quantity that sufficient compressive strength cannot be obtained in a reasonable period of time. Such compressive strength is generally considered to be about 500 psi to permit drilling out and the reasonable period of time is considered to be approximately 24 hours. Additionally, organic acids, such as gluconic acid, act as strong retarders and are temperature sensitive. Such organic acids can only be used when the well formation temperature is sufficiently high so as to overcome such retarder characteristics. Such minimum BHCT is about 200.degree. F.
U.S. Pat. No. 4,340,525 discloses certain copolymers and terpolymers for use in deep-well cementing operations. Such copolymers comprise 1-60 mole percent of anionic structural units and about 99-40 mole percent of nonanionic structural units. The terpolymers further include 10 mole percent of a water soluble monomer selected from the group consisting of vinyl sulfonic acid, vinylpyrolodone, esters of amino alcohols and acrylic acids, esters of amino alcohols and methacrylic acid and acrylic acid, amidosulphonic acid. A preferred terpolymer includes acrylamide, sodium acrylate and sodium vinylsulfonate. However, such compositions when used in conjunction with fresh water cement slurries tend to form stiff gels at a temperature above 200.degree. F. Additionally, such terpolymers when used in saturated salt cement slurries tend to lose effectiveness at temperatures well below 400.degree. F. This is also due to the requirement that large amounts of the terpolymer be included to effect retardation thereby preventing the development of sufficient compressive strength within a reasonable period of time after placement.
Hence, the industry desires a cement retarder that will retard the set time of cement under high temperature (above 400.degree. F.) conditions for a sufficient period of time so as to permit placement of such cement during cementing operations, permit the development of adequate compressive strength within a reasonable period of time (24 hours), is salt tolerable (i.e. does not exhibit substantial loss of effectiveness in the presence of salt), is chemically stable during cementing operations, and has as little effect on viscosity as possible. Such a cement retarder should be compatible with as many other additives and environmental conditions as possible and should be soluble in cement slurries at normal ambient temperature conditions encountered during oil well cementing operations.